Direct burning of wet cellulose sludge is inefficient and produces unacceptable emissions which are harmful to the atmosphere. A process of drying dewatered cellulose sludge to a 15% water content prepares the sludge that is more suitable for burning. With this drying procedure, combustible gases are supplied for the self-operation of the dryer and for excess use that can be taken off for steam generation and other uses. The final burning converts the dried material to ashes, thus reducing the amount of sludge going to landfills.
It has long been recognized that landfills have posed a great problem because they can contaminate ground water and the atmosphere about them. Accordingly, federal and state regulations have placed many restrictions on their design and maintenance resulting in substantial costs to industry and communities, if, and even when, a suitable site has been found.
From wastewater treatment plants the sludge is dewatered with mechanical presses to reduce the overall water content before it is transported to the landfill, The building up of the landfill, trucking, bulldozer and maintenance can run to $100.00 per ton and more.
Furthermore, the mechanical pressing equipment is expensive to install, operate and maintain. One such press is in the form of a tapered screw operable in a tapered perforated chamber of high structural strength. The sludge is forced along in ever-decreasing space while water under much more pressure is driven through the perforations. The moisture content is reduced to some 40% and considerable horsepower is consumed in the process. In carrying out the present process, this pressed sludge is supplied in its dewatered cake state and after being crumbed and metered the sludge is delivered to a travelling wire screen conveyor and passed through drying chamber assemblies.
An attempt has been made by pulp and paper mills to burn the cellulose sludge. Because the wood fiber will not ignite with a moisture content slightly above 15%, much energy is consumed in the ordinary burning to evaporate excess moisture and the process becomes most inefficient. Burning wet sludge damages the boiler tubes as the products of combustion adhere to the tubes creating blockages and causing increased velocity between the boiler tubes that abrade and erode the tubes.
Tumble dryers have been used with some success, but they lack the control that is necessary to regulate the temperature for the drying of the sludge, thereby resulting in overdrying the sludge and creating an explosive situation due to airborne particles being about.
Hence, there are many disadvantages to current drying methods, such as lack of adequate temperature control, the need for outside fuel, the need to adjust for characteristic changes of the incoming sludge and low production capacities result.